Cells: Iteration 2: Introduce `CrossDatabaseIgnoredTables` helper

Problem

As a temporary violation allowance we need to allow modifying tables (assigned to different schemas) while saving active record models. Without that it will trigger cross-DB violations that are not fixable at the current moment. To allow us to gracefully handle cross-DB violations by marking them, creating issues, getting pipeline green, and fixing them later.

Example case:

• User does create personal namespace on user creation
• Ideally, User model should create personal namespace asynchronously. Today this is done in-sync.

Proposal

• Introduce cross_database_ignore_tables helper method that will temporarily ignore cross-DB violations for a given tables on a given callback conditions, while requiring to pass url: (similar to allow_cross_joins_across_databases)

Example usage:

class User
  # allow namespaces to be created within a transaction that does violate cross-DB
  cross_database_ignore_tables %w[namespaces], on: [:create, :destroy], url: "http://TODO"
  cross_database_ignore_tables %w[namespaces], on: [:update], if: -> { namespace&.changed? }, url: "http://TODO"
end

References

• Draft: Cells: Move `users` and `namespaces` in ... (!108462 - closed)